Generally, infrared light in a long wavelength range of 3 μm or more is used for human sensors for detecting the presence of a human body, noncontact temperature sensors, gas sensors and the like because of its thermal effect or infrared light absorption effect by gas. Among these application examples, particularly gas sensors have received attention in recent years because they are usable for monitoring and protecting air quality and also for early detection of fire and the like. The principles of the gas sensors using the infrared light are as follows. First, a gas to be measured is injected into a space between an infrared light source and a light receiving device. Since a particular gas absorbs infrared light having a specific wavelength, the type and concentration of the gas can be measured by analyzing wavelength spectrums before and after the injection of the gas. Here, an incandescent bulb is used as the infrared light source, but infrared light emitted from the incandescent bulb is white light. Thus, in order to disperse the light having a specific wavelength, a filter needs to be provided on the light receiving device side. Such a filter is expensive. In addition, the filter lowers the intensity of the infrared light, and consequently lowers the sensitivity of the gas sensor. Moreover, since the incandescent bulb has a short life, the light source needs to be replaced frequently.
To solve the problems described above, it is effective to use, as the light source, a semiconductor light emitting device (LED: Light Emitting Diode) which emits infrared light having a specific wavelength. Here, in order to produce such a light emitting device, a device which emits infrared light in a long wavelength range of 3 μm or more is required. However, the device with such a wavelength range is greatly affected by an ambient temperature, and has a trouble in use at room temperature. In general, in the light emitting device described above, a so-called pn junction diode structure is formed in a semiconductor having a band gap that allows emission of infrared light with a wavelength of 3 μm or more. The light emitting device emits infrared light by causing a forward current to flow through the pn junction diode, and then recombining electrons and holes in a depletion layer that is a junction portion.
However, the band gap of the semiconductor which allows emission of infrared light with a wavelength of 3 μm or more is as small as 0.41 eV or less. Such a semiconductor having a small band gap cannot obtain sufficient pn diode characteristics because thermally excited carriers increase an intrinsic carrier density at room temperature and thereby reduce a device resistance. This is because, when the intrinsic carrier density is high, a leak current of the device, such as a diffusion current and a dark current, is increased. For this reason, in such a light emitting device, a cooling mechanism such as a Peltier element is generally and conventionally used to suppress thermally excited carriers. However, there is a problem that such a cooling mechanism increases the size and price of the device.
To solve the above problem, research and development have been conducted on a light emitting device capable of emitting infrared light in a long wavelength range even at room temperature. For example, in a light emitting device described in Non-patent Document 1, a diode having a p-n-n structure with indium antimonide (InSb) is formed on a p-type InSb substrate, and an AlInSb barrier layer for suppressing diffusion of electrons is used between the p layer and the π layer to enable emission of infrared light with a wavelength of 5.5 μm or more at room temperature.
In terms of a conventional semiconductor material having a small band gap, emphasis is placed on suppressing a leak current (diffusion current or dark current) of electrons as described in Non-patent Document 1, because electron mobility is generally much higher than hole mobility. However, in the light emitting device which recombines electrons and holes, suppression of dark current and diffusion current of holes in addition to electrons is required to further improve device characteristics.
The present invention has been made in consideration of the foregoing problems. It is an object of the present invention to provide an infrared light emitting device capable of suppressing diffusion current and dark current of thermally excited holes at room temperature.
Non-patent Document 1: T. Ashley et al., “Uncooled InSb/Inl-XAlXSb mid-infrared emitter,” Applied Physics Letters, 64(18), 2 May 1994, pp. 2433-2435